So today, as I was sitting at my desk, I came across this rather fantastic, but simple build by the circuit.io team. It's an air gate sensor which you can use for drone races, or any form of object detection really. It uses an ultrasonic sensor and some simple logic to determine whether or not an object has passed close to it. If so, then it gives feedback via an RGB LED, informing the pilot that they successfully passed the air gate.

After checking it out, I loved the simplicity of the project, but my mind immediately jumped to the possibility of adding RFID readers to it, and tiny RFID tags to the drones, which would allow the gates to tag the exact drone that passed by it, giving the user all kinds of data such as time between gates, proximity to other drones, and more advanced tracking abilities.